A known heat exchanger, such as a radiator, has a radiator core including tubes and fins, which are alternately stacked in its vertical direction. Such a radiator core is interposed between side tanks in its lateral direction. Such a side tank includes a tank member and a base member, which are assembled together to define an inner cavity to flow a thermal medium therethrough. Conventionally, such a tank member and a base member are crimped together at both lateral peripheries by applying force using a crimping apparatus, such as a die and/or a roller. In such a conventional side tank, large working spaces are needed around both the lateral peripheries of the tank to place the crimping apparatus on each of the lateral peripheries of the tank. In addition, complicated manufacturing process is required to crimp both the lateral peripheries of a conventional side tank.